feat(storage): expose object_metadata on AsyncMultiRangeDownloader (#17411)

This PR exposes the `object_metadata` attribute on the
`AsyncMultiRangeDownloader` class.

### Context
In PR #17261, `object_metadata` was successfully exposed on the
`AsyncReadObjectStream`. However, it was not surfaced on the
`AsyncMultiRangeDownloader` (MRD), meaning high-level libraries relying
on MRD (like `fsspec`/`gcsfs`) cannot access the underlying gRPC
metadata.

By inheriting and surfacing `self.object_metadata =
self.read_obj_str.object_metadata`, this allows downstream users to
completely bypass redundant `REST` `info()` API calls and extract fields
like `crc32c`, `size`, and `generation` directly from the MRD instance.

cc @chandra-siri

---------

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: yuxin00j

packages/google-cloud-storage/google/cloud/storage/asyncio/async_multi_range_downloader.py

@@ -565,3 +565,8 @@ async def close(self):
     @property
     def is_stream_open(self) -> bool:
         return self._is_stream_open
+
+    @property
+    def object_metadata(self) -> Optional[_storage_v2.Object]:
+        """The metadata of the object being downloaded."""
+        return self.read_obj_str.object_metadata if self.read_obj_str else None

packages/google-cloud-storage/tests/unit/asyncio/test_async_multi_range_downloader.py

@@ -61,6 +61,7 @@ async def _make_mock_mrd(
         mock_stream.generation_number = _TEST_GENERATION_NUMBER
         mock_stream.persisted_size = _TEST_OBJECT_SIZE
         mock_stream.read_handle = _TEST_READ_HANDLE
+        mock_stream.object_metadata = mock.Mock()
 
         mrd = await AsyncMultiRangeDownloader.create_mrd(
             mock_client, bucket_name, object_name, generation, read_handle
@@ -94,6 +95,7 @@ async def test_create_mrd(self, mock_cls_async_read_object_stream):
         assert mrd.read_handle == _TEST_READ_HANDLE
         assert mrd.persisted_size == _TEST_OBJECT_SIZE
         assert mrd.is_stream_open
+        assert mrd.object_metadata == mrd.read_obj_str.object_metadata
         assert mrd._open_retries == 0
 
     @mock.patch(
@@ -276,6 +278,7 @@ async def test_close_mrd(self, mock_cls_async_read_object_stream):
 
         # Assert
         assert not mrd.is_stream_open
+        assert mrd.object_metadata is None
 
     @pytest.mark.asyncio
     async def test_close_mrd_not_opened_should_throw_error(self):